In recent years, hydrogen has attracted a great deal of attention worldwide as a clean energy source and from the perspective of diversification of energy. In particular, expectations are high for fuel cell vehicles using hydrogen gas as a fuel source, research related to the development of fuel cell vehicles has been widely progressed all over the world, and some fuel cell vehicles have already been put to practical use.
Fuel cell vehicles run with a hydrogen gas tank on-board, instead of on gasoline. Thus, development of hydrogen stations on behalf of gas stations is essential for the spread of fuel cell vehicles. In hydrogen stations, hydrogen is stored in a hydrogen pressure vessel, which is a container for high-pressure hydrogen, and hydrogen is charged from the pressure vessel to a hydrogen tank mounted on the fuel cell vehicle.
In order for a fuel cell vehicle to achieve a cruising distance comparable to that of a gasoline-powered vehicle, it is necessary to set the maximum filling pressure of the hydrogen tank mounted on the fuel cell vehicle to 70 MPa. It is thus required for an on-board hydrogen tank to be able to safely store and supply hydrogen under such a high-pressure hydrogen environment.
Similarly, in order to set the maximum filling pressure of the on-board hydrogen tank to 70 MPa, it is necessary to increase the maximum filling pressure of the hydrogen pressure vessel used at a hydrogen station to 82 MPa. From this follows that the hydrogen pressure vessel in the hydrogen station will be exposed to an ultra high-pressure hydrogen gas environment as high as 82 MPa.
Candidate materials to be used under such a high-pressure hydrogen environment include low alloy steel, stainless steel, aluminum alloy, plastic, and the like. However, when a steel material is used in a hydrogen environment, the strength decreases (hydrogen embrittlement) and, especially in low alloy steels, the drawability decreases.
Hydrogen embrittlement of a material occurs when hydrogen taken from the surrounding environment diffuses into the material and accumulates in a stress concentration part, a grain boundary, or the like. Therefore, the study of hydrogen permeation behavior in materials is very important, and research has thus flourished. For example, Takahiro Kushida, “Materials and Environment”, Vol. 49 (2000), pp. 195-200 (NPL 1) describes an electrochemical hydrogen permeation method for investigating the behavior of hydrogen generated in solution permeating through a material. In addition, JPS54104551A (PTL 1) and JPH03047706B (PTL2) describe measurement of hydrogen permeation behavior in a material under a hydrogen gas environment.